the particular miner who is mining the block chooses what transactions to add to the block and also puts in the address for where they want the block reward to go to.

core doesnt really do the mining (block consolidation/creation) side of things any more its just validates a received block follows the rules.

the block consolidation/creation code is usually in mining pool software which is different to the main core code

look at the 'blocktemplate' for where it goes (getblocktemplate)

oh.
well bloating the blockchain with meaningless data is not really a good thing to do
no one is going to happily advertise how to a newbie. otherwise everyone would be doing it

Most miners don't actually use getblocktemplate anymore and they have their own software written to accommodate for their needs (ASICBoost, custom msg in coinbase etc).

The documentation is here: https://developer.bitcoin.org/reference/rpc/getblocktemplate.html. The code is written here: https://github.com/bitcoin/bitcoin/blob/793e0ff22cbace2a0fbe1e4a2e88a7bc6bf44502/src/rpc/mining.cpp#L488.